One of the most significant developments in patient temperature measurement in recent years has been that of the non-contact infrared (IR) thermometer such as that disclosed in U.S. Pat. No. Re: 34,789 to Fraden, the contents of which are incorporated herein by reference. The IR thermometer typically includes a sensitive infrared detector and an optical system that guides the radiation from the tympanic membrane of the patient's ear to the sensor for measurement. The sensed radiation is converted to a temperature reading that is closely correlated to the patient's temperature. Importantly, the measurement is completed within one or two seconds and with minimal discomfort to the patient.
Particularly in a hospital or clinic setting, care must be taken to prevent the spread of infectious agents and cross-contamination of patients whose temperatures are taken on a periodic basis by the use of an IR thermometer. In addition, the optical element of the infrared thermometer must remain clear of ear wax in order to operate reliably. To meet these concerns, a protective cover is almost always placed over the IR thermometer sensor probe to provide a sanitary barrier that precludes direct physical contact between the IR sensor and the patient's ear, and which is typically discarded and replaced with a new probe cover after each use.
Previously developed clinical thermometers that come into contact with mucous membranes to sense temperature also required the use of a disposable cover as a sanitary barrier. These prior sanitary covers typically included a rigid wall and a thin film contiguous with the contact sensor element that was often stretched to minimize the thermal barrier to conductive heat transfer. Exemplary early thermometer covers of this type are described in U.S. Pat. No. 3,822,593 to Oudewaal and U.S. Pat. No. 3,987,899 to Vyprachticky.
Disposable covers, which have been employed with the more recently developed infrared thermometer, have generally been closely modeled on those that were used with the earlier contact thermometers. For example, U.S Pat. Nos. 5,293,862 and 5,179,936 to O'Hara et al. disclose a two-piece protective disposable cover for an infrared thermometer in which a thin transparent film is bonded to a rigid tubular body. The manufacturing process of this cover, however, may cause the formation of wrinkles in the thin transparent film that could interfere with infrared transmission between the tympanic membrane of the ear and the IR sensor of the thermometer. To ensure that no wrinkles are formed, the transparent film has been stretched over the infrared sensor.
However, stretching the film to prevent the formation of wrinkles has been found to create other sources of potential measurement inaccuracies. First, a film that is stretched may stretch in a non-uniform manner creating a "lensing" effect that could distort transmitted infrared radiation. Second, stretching the film may result in a realignment of the polymer molecular structure that could cause variations in both the reflective and absorptive properties of the film. Accordingly, these infrared probe covers could display unpredictable transmission properties when they were fitted onto a probe of an infrared thermometer.
U.S. Pat. No. 4,911,559 to Meist and Suszinski discloses an infrared probe cover that is also susceptible to the effects of stretching the film when the cover is placed over an infrared thermometer probe. The effects caused by this stretching of the film adversely affected the transmissivity of the film to IR energy in unpredictable ways that often resulted in errors in temperature measurement.
Other prior art infrared thermometer probe covers were often configured so that the transparent thin film came into contact with the patient's skin. If the patient's skin is at a different temperature than that of the probe cover, a temperature gradient is formed in the film as heat conduction triggers an energy flow from the warmer ear to the cooler film. The resulting elevated temperature of the film amplifies an error source known as "secondary radiation", which refers to the infrared radiation that the cover emits relative to the infrared radiation emitted by the primary source, viz. the tympanic membrane of the ear. Such secondary radiation may introduce temperature measurement inaccuracies.
U.S. Pat. No. 5,179,936 describes a disposable sanitary probe cover for use with an infrared thermometer in which a plastic film or membrane, which serves as the IR window, is stretched across the front end of the tubular body portion of the disposable speculum. The fabrication of this device, however, requires precise control in order to maintain proper performance and the correct degree of stretch, and is accordingly relatively costly. In addition, this cover is only suitable for a single use, which makes it unacceptable for use in the international consumer markets.
An improved unitary disposable protective cover for use with an infrared thermometer is disclosed in U.S. Pat. No. 5,795,067 to Fraden, et al. and U.S. Pat. No. 5,088,834 to Howe, et al., the contents of which are incorporated by reference as if restated in full. The cover disclosed in the '067 patent includes an IR window secured to a thin sidewall by means of a collar structure which either isolates the film from externally applied forces or develops counter forces in a way that precludes stretching of the film and other effects that would disturb IR transmission through the window. While this protective cover provides a significant advantage over prior probe covers, there are several issues relating to its use.